Punnett Squares Lesson

Punnett Square Diagram Image, explanation of diagram found in page text The diagram to the right is called a Punnett Square. It is used to predict the outcome of a particular cross or breeding experiment just like Mendel did with the pea plants at the monastery. The Punnett Square to the right shows a cross between a homozygous green pea pod plant and a heterozygous yellow pea pod plant. The yellow pea pod color is the dominant trait. It has a capital letter Y which represents the dominant allele for the gene that controls pod color. The lowercase y represents the recessive allele for pod color which is green. Can you tell how many plants from this cross will have a green pod color? Although two of the pea plants have the alleles Yy, recall that Y is the allele for the dominant trait. When the dominant allele is present, its trait will show up in the organism. If the dominant allele is not present, then the organism has two recessive alleles and the trait for the recessive allele will show up or will be expressed. Two out of the four offspring in the cross inherited the recessive allele, y, from each parent. Therefore, ½ of the pea plants will have a green pod color. As the Punnett Square depicts, two recessive alleles are written as yy. Since the alleles are the same, they are called homozygous. Both alleles are also recessive. The green pod pea plants in this cross can also be described as homozygous recessive. An organism is heterozygous for a particular trait if it has alleles that are different for a gene. A hybrid is also a heterozygous organism. Based on the Punnett Square above, 1/2 of the pea plants will have a yellow pod color. Their genotype or combination of alleles is Yy.What is Mendel's principle of dominance? What is Mendel's law of segregation? What is a monohybrid cross? Click on the presentation to answer these questions and learn how to construct a Punnett Square.

The following activity, adapted from the Dolan DNA Learning Center, illustrates how a tool developed by scientist Reginald Punnett — the Punnett square — validates Gregor Mendel's laws of genetic inheritance. Watch how the Punnett square can be used to display the possible genetic outcomes when two yellow-seed hybrid pea plants are crossed. The results clearly illustrate Mendel's principle of dominance and why the potential offspring traits appear with a predictable 3-to-1 dominant-to-recessive frequency.

Suppose your friend has a pet chicken and wants to breed it so that she can raise the chicks. She has found a rooster for her hen to breed with, but she wants to know what their chicks will look like before she breeds them! How can we predict what the offspring of her hen and this rooster will look like? We need to know what outcomes are possible. In order to figure out all the possible genotypes that can result from crossing two organisms, scientists sometimes use a tool called a Punnett square.

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